Much effort is invested worldwide for developing storage for trough technology. The more advanced approach is based on phaze changes materials (which is called: PCM), since it enables higher density in the storage and minimal temperature losses between charge and discharge. The main problem is the low heat transfer (due to low thermal conductivity of the salts), and this affects directly the amount of power that could be extracted from the storage. Several research is being executed (mainly in Germany) for developing enhanced solutions, usually by enhancing the heat transfer between the salt and the heat transfer fluid (in the molten salt receiver/hot storage tank), reducing transient effects, optimization of the storage materials (for example, by using metal with graphite that has very high thermal conductivity – which can result up to 15% increase in conductivity, modifications in geometry, boundary conditions (e.g., addition of inflow and outflow, adding radiating surfaces or media) are being tested. Parts of those solutions are technically feasible, although too expensive yet, e.g. the additional costs overweigh the benefits. One of the advanced approaches, that might have a chance to be cost effective, is based on adding metal surfaces into the salt zone, which may significantly improve the heat transfer to the salt by adding both radiative and convective areas, and also induce more mixing by producing faster flow and higher turbulence. Another alternative to these effects is to add particles that participate in radiation and supply convection area. The goal is to achieve an energy storage system with thermal efficiency of 90%, life time of 30 years and specific costs of: 30 USD/KWthermal capacity, and 1.5 US cent per KWHelectric. But, as far as I know, no system can achieve it yet.
Various storage systems incorporated with solar tower electricity generation systems were developed and the most advanced of them was installed and tested in California. This system, Solar Two, generated 10MW electricity using an eutectic molten nitrate salts mixture pumped and piped from a ground-based cold tank to a receiver mounted on the top of a tower. The hot salt from the receiver is then piped to a second, hot tank on the ground. In a secondary loop, the hot salt flows through a heat exchanger to generate steam and returns to the cold tank. The third loop includes the steam generator, which supplies steam to a steam turbine electricity generator. This plant was closed on 1999. Now Sener is trying to do something similar in Spain.
The most common storage technology in use (following the inefficient oil storage tanks solution that is being used at the SEGS plants in California) is the molten salt two-tank system, which provides a feasible storage capacity and is considered to have low to moderate associated risks. Molten salt that will be used for storage as such is bankable (as molten salt is being used for a long time in the chemical industry), but the integration of this kind of storage system to the solar system – is risky. Concentrated solar thermal power plants have specific requirements for storage that are not well known in the chemical industry. For example: working under thermal cycling conditions; heating and cooling; temperature changing periodically; even design the hot and cold tanks is a challenge; Not to speak about the pipes and the heat exchangers. Another problem is freezing at night. But the main risk is that it is a big step from the existing technology in the chemical industry to that is required by the solar plants, especially in size – going up in scale, since in the chemical industry relatively small amounts of molten salts are being used. On the other hand, storage contributes not only by increasing operation hours, but also enhancing the overall efficiency, as the plant is working more hours close to the design point.
At Acciona’s Nevada parabolic trough plant there is no storage (only for about 30 minutes, which is achieved by the fluid that is in the pipes). On the other hand, at the parabolic trough plants of Andasol One & Two – FlagSol (Solar Millennium’s subsidiary) together with ACS/Cobra developed thermal storage based on molten salt. This system is being working for almost two years, probably with a lot of obstacles to deal with, like freezing issues (the freezing point of the chosen nitrates is probably 220ºC), corrosion, blocking, purity of the salt, problems with materials that are in contact with the salt, and a lot of integration and control issues. However, the operators (ACS/Cobra) are gaining much experience and claim to be able to overcome most obstacles.
Another risk related to incorporating storage is how much downtime (forced outage) will the plant experience. As a worth case scenario one has to assume up to 10 percent (36.5 days) down, although some plants have almost no downtime due to troubles with storage systems. Thermal storage allows project developers to maximize the value of the solar thermal facility’s output for time of use pricing verses the cost of producing that electricity. Designing a facility to sell the largest amount of output does not necessarily make that design the one with the best return on capital. Sometimes it is preferred, for example, to store all of the thermal energy produced in the morning instead of directing only part to the storage and part to produce electricity for immediate sale. The design point as well as operation strategies are of utmost importance especially when thermal storage is incorporated with the solar thermal plant. However, reducing drastically the capital cost of thermal storage is key to the commercial deployment of the technology.
